1. Field of the Invention
The present invention relates to an improved quality control method for laser shock processing in which the consistency of residual stress effects produced thereby are monitored and more specifically, relates to a quality control method which employs bend bars for such testing.
2. Description of the Related Art
Laser peening creates deep compressive residual stresses in the surface of a metal part being treated. Such compressive stresses beneficially improve the fatigue strength, fatigue life, fretting fatigue resistance, and stress corrosion cracking resistance of components.
There are problems associated with monitoring and verifying the level of residual stresses developed in a part. Residual stress measurements may be made using X-Ray diffraction (XRD) measurements or compliance methods (e.g., hole drilling, strain gauging with sectioning, etc.), but such methods are generally destructive in that samples must be sacrificed for testing. In addition, these tests are time consuming, and the results are not available until after all the parts are processed. During laser peening operations, it is desirable to perform periodic quality checks to ensure that the application of laser peening is consistent and reproducible. Toward this goal, it is useful to have a test specimen that may be processed quickly and economically to assess the status of the laser peening operation. Such a test specimen and method may also be used for process development and optimization.
Such a test specimen that has been used to ascertain the extent of traditional shot peening coverage is a small, thin steel strip known as an Almen gage or Almen strip. There are three different thicknesses of these strips, 0.040, 0.060, and 0.090 inches thick denoted as N, A, and C, respectively. The selection of the strip is dependent upon the intensity of the shot peen process to be used on the components. The thinner strips are used for relatively low intensity shot peening, while the thicker strips are used for high intensity shot peening. The strips are shot peened on one side causing the strip to bow toward the peened surface as the residual stress increases. As such, the extent of bowing at the center of the strip is a relative measure of the induced compressive residual stress and the extent of coverage. The bowing is induced in the strip because of the compressive residual stresses induced to a depth of nominally 0.005 inches near the surface of the strip. The use of Almen strips to monitor laser peening has been attempted, but the results have been disappointing with considerable data scatter and poor reproducibility.
A specific drawback with the use of a standard steel Almen strip for use with laser peening is the limited thickness thereof. The thickest commercially available Almen strips, C strips, are only 0.090 inches in thickness. Since the compressive residual stresses generated with laser peening are often greater than 10 times deeper than those generated with shot peening, the residual compressive stress profiles produced in steel by laser peening can extend to 0.040 inches to 0.060 inches deep, thereby extending past the mid-plane thickness of the steel strip. It has been realized that when the compressive residual stress profile extends past the mid-plane of the strip, there would be a reverse bending component driven by the compressive residual stresses extending past the mid-thickness of the strip. In this case, there would not be a single strip arc-deflection value for each laser peening intensity. This characteristic of the Almen strips make them unsuitable for monitoring laser shock processing.
Clauer® strips have been developed as a means to monitor laser peening operations. These are thin aluminum alloy 6061-T6 strips (100 mm×6 mm×t), where the thickness is selected within the range of 0.9 to 2.0 mm, depending upon the laser peening conditions being applied. Clauer strips are one-shot, mechanical gages that measure the impulse generated by the laser pulse. The strips are gripped at one end, and the laser pulse hits on the opposite end. The intensity of the laser pulse is related to the amount of bending in the Clauer® strip.
While the Clauer® strips are relatively inexpensive and easy to use, the amount of bending is not necessarily a true measure of the residual stresses induced in the part, but rather a measure of the force applied to the tip of the strip. Alignment of the gage is rather critical, and moderate scatter in the data can occur depending on a number of factors. As such, Clauer® strips are probably best used as a “go-no go” semi-quantitative test rather than as a direct indicator of residual stresses imparted to the workpiece.
What is needed in the art is a reliable and economical quality control tool to measure the effects of compressive residual stresses developed during a laser peening process to ensure product consistency in laser peened components during production operations.